1. Field of the Invention
The present invention relates to a semiconductor device typified by a thin film transistor and to a fabrication method thereof. The present invention also relates to a semiconductor device using a crystal silicon thin film formed on a substrate such as a glass substrate and quartz substrate and to a fabrication method thereof. Further, the present invention relates to an insulated gate type semiconductor device such as a thin film transistor and to a fabrication method thereof.
2. Description of Related Art
Hitherto, there has been known a thin film transistor using a silicon film, i.e. a technology for forming the thin film transistor by using the silicon film formed on a glass substrate or quartz substrate. The glass substrate or quartz substrate is used as the substrate because the thin film transistor is used for an active matrix liquid crystal display. While a thin film transistor has been formed by using an amorphous silicon film (a-Si) in the past, it is being tried to fabricate the thin film transistor by utilizing a silicon film having a crystallinity (referred to as xe2x80x9ccrystal silicon filmxe2x80x9d hereinbelow in the present specification as necessary) in order to enhance its performance.
The thin film transistor using the crystal silicon film allows to operate at a high speed by more than two digits as compared to one using the amorphous silicon film. Accordingly, while peripheral driving circuits of an active matrix liquid crystal display have been composed of external IC circuits, the crystal silicon film allows them to be built on the glass substrate or quartz substrate similarly to the active matrix circuit. Such structure is very advantageous in miniaturizing the whole apparatus and in simplifying the fabrication process, thus leading to the reduction of the fabrication cost.
Hitherto, a crystal silicon film has been obtained by forming an amorphous silicon film by means of plasma CVD or low pressure thermal CVD and then by crystallizing it by implementing a heat treatment or by irradiating laser light. However, it has been the fact that it is difficult to obtain a required crystallinity across the wide area through the heat treatment because it may cause nonuniformity in the crystallization. Further, although it is possible to obtain the high crystallinity partly by irradiating laser light, it is difficult to obtain a good annealing effect across the wide area. In this case, the irradiation of the laser light is apt to become unstable under the condition for obtaining specifically a good crystallinity.
By the way, the inventors et. al. have developed a technology for obtaining the crystal silicon film through a heat treatment at a lower temperature than that of the prior art by introducing a metal element (e.g. nickel) which promotes the crystallization of silicon to the amorphous silicon film (Japanese Patent Laid-Open Nos. Hei. 6-232059 and Hei. 7-321339). These methods allow not only the crystallization speed to be increased and the crystallization to be achieved in a shorter time, but also a high crystallinity to be obtained uniformly across the wide area, thus having a crystallinity which fits for practical use, as compared to the prior art crystallization of amorphous silicon film implemented only by way of heating or by way of the irradiation of laser light.
However, because the metal element is contained within or on the surface of the crystal silicon film, the amount thereof to be introduced has to be controlled very carefully, thus posing a problem in its reproducibility and stability (electrical stability of a device obtained). Specifically, there is a problem that an elapsed change of the characteristics of a semiconductor device to be obtained is large or an OFF value, in case of a thin film transistor, is large, due to the influence of the remaining metal element. That is, although the metal element which promotes the crystallization of silicon plays the valuable and useful role in obtaining the crystal silicon film, its existence becomes a minus factor which causes various problems after obtaining the crystal silicon film once.
Then, after conducting a large number of experiments and discussions from various aspects in order to solve the problem in forming the crystal silicon film by introducing the metal element (e.g. nickel) which promotes the crystallization of silicon to the amorphous silicon film and by treating by heat as described above, the inventors et. al. have found that the metal element contained and remaining in the crystal silicon film may be eliminated or reduced by the specific method described later, thus reaching to the present invention.
By the way, because an active matrix liquid crystal display is small, is light and is able to display fine motion pictures at high speed, it is being expected to become the mainstream of displays of the future. However, because it has a limit that a substrate composing the liquid crystal display needs to be translucent, its type is limited. A glass substrate, a quartz substrate or a plastic substrate may be cited as an example thereof.
However, among them, the plastic substrate has a problem that it lacks in heat resistance and the quartz substrate has a problem that it is very expensive and its cost is more than 10 times of the glass substrate especially when it is widened, thus lacking in cost performance, though it can withstand a high temperature of about 1000xc2x0 C. or 1100xc2x0 C. Accordingly, the glass substrate is widely used general from the reasons of heat resistance and economy.
Currently, the performance required for the liquid crystal displays is getting higher and higher and the performance and characteristics required for a thin film transistor (hereinafter referred to as a TFT as necessary) used as a switching element of the liquid crystal displays is also getting higher. Due to that, while the research and development for forming the crystal silicon film having the crystallinity on the glass substrate are being actively conducted, the crystal silicon film is formed on the glass substrate by adopting the method of forming the amorphous silicon film and of crystallizing it by treating by heat or by irradiating laser light at the present.
That is, because the heat resistant temperature of the glass substrate is normally about 600xc2x0 C., though it depends on a type thereof, a process which exceeds the heat resistant temperature of the glass substrate cannot be adopted in the step for forming the crystal silicon film. Therefore, a method for forming the amorphous silicon film by means of plasma CVD or low pressure CVD and crystallizing it by heating at a temperature below that heat resistant temperature has been adopted in forming the crystal silicon film on the glass substrate. The method of crystallizing the silicon film by irradiating laser light also allows the crystal silicon film having an excellent crystallinity to be formed on the glass substrate and has an advantage that the laser light will not damage the glass substrate thermally.
However, the crystal silicon film crystallized from the amorphous silicon film by the above-mentioned technologies has had a large number of defects caused by dangling bond and the like. Because these defects are the factor of degrading characteristics of the TFT, it is necessary to passivate the defects at the interface between an active layer and a gate insulating film and the defects within and at the boundary of the crystal grains of the silicon of the active layer in fabricating the TFT by utilizing such crystal silicon film. The defects at the grain boundary in particular are the greatest factor of scattering charge, but it is very difficult to passivate the defects at the grain boundary.
Meanwhile, it is possible to compensate the defects at the grain boundary of the crystal silicon film by Si in fabricating a TFT on the quartz substrate because it is possible to implement a heat treatment at a high temperature of about 1000xc2x0 C. or 1100xc2x0 C. for example. In contrary to that, it is difficult to implement the heat treatment in high temperatures in fabricating the TFT on the glass substrate, so that the defects of the grain boundary of the crystal silicon film are passivated by hydrogen by implementing a hydrogen plasma treatment in an atmosphere of about 300 to 400xc2x0 C. normally in the final stage of the process.
An n-channel type TFT presents a practical field-effect mobility by implementing the hydrogen plasma treatment. On the other hand, the effect of the hydrogen plasma treatment is not so remarkable in a p-channel type TFT. It is construed to happen because a level caused by the defect of the crystal is formed in a relatively shallow domain under a conduction electron zone. Although it is possible to compensate the defect of the grain boundary of the crystal silicon film by implementing the hydrogen plasma treatment, an elapsed reliability of the TFT or the n-channel type TFT in particular which has been treated by the hydrogen plasma is not stable because the hydrogen compensating the defect is apt to be desorbed. For instance, if the n-channel type TFT is energized for 48 hours in an atmosphere of 90xc2x0 C., its mobility is reduced to a half.
Further, although the quality of the crystal silicon film obtained by irradiating laser light is good, ridges (irregularity) are formed on the surface of the crystal silicon film if the thickness of the film is less than 1000 angstrom. when laser light is irradiated to the silicon film, the silicon film is melt instantly and expands locally. The ridges are formed on the surface of the crystal silicon film to relax internal stress caused by this expansion. A difference of elevation of this ridge is about xc2xd to 1 time of the thickness of the film. For instance, when laser annealing is implemented after crystallizing an amorphous silicon film whose thickness is about 700 angstrom by way of heating, ridges of 100 to 300 angstrom in height are formed on the surface thereof.
Because a potential barrier and a trap level caused by the dangling bond, distortion of lattice and the like are formed at the ridges on the surface of the crystal silicon film in an insulated gate type semiconductor device, the level of the interface between the active layer and the gate insulating film becomes high. Further, because the peak of the ridge is sharp and thus an electric field is apt to concentrate there, it may become a source of leak current, causing dielectric breakdown in the end. Further, because the ridge on the surface of the crystal silicon film damages a coating quality of the gate insulating film deposited by way of sputtering or CVD, it degrades the reliability of insulation by causing defective insulation.
Accordingly, it is an object of the present invention to provide a novel and very useful method for forming a crystal silicon film by introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and for eliminating the metal element or for reducing the concentration of the metal element within the crystal silicon film thus obtained.
It is another object of the present invention to provide a semiconductor device having excellent characteristics, and a fabrication method thereof, fabricated by using a crystal silicon film having a high crystallinity and obtained by introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and by eliminating the metal element or by reducing the concentration of the metal element in the crystal silicon film.
It is a further object of the present invention to provide a semiconductor device, and a fabrication method thereof, which allows the characteristics and reliability of the semiconductor device thus obtained to be enhanced.
It is still another object of the present invention to solve the aforementioned problem by providing a method for fabricating a semiconductor device which allows the defects at the crystal boundary of the silicon film crystallized from the amorphous silicon film to be passivated without using the hydrogen plasma treatment.
It is another object of the present invention to provide a method for fabricating a semiconductor device having a high reliability and high mobility and more particularly to provide a semiconductor device, and a fabrication method thereof, which has a gate insulating film composed of deposited films, which is formed on a glass substrate and whose reliability and characteristics are enhanced.
While the present invention has objects, beside those described above, which correspond to the structures described below, these will be explained as necessary complementarily in the description which follows.
In order to solve the aforementioned problems, the present invention possesses the following aspects:
(1) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere; eliminating a thermal oxide film formed in the previous step; and forming a thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(2) The present invention provides a method for fabricating a semiconductor device, comprising-steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere to form a thermal oxide film on the surface of the crystal silicon film and by causing the thermal oxide film to getter the metal element; eliminating the thermal oxide film formed in the previous step; and forming a thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(3) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second oxidation heat treatment within an oxidizing atmosphere; eliminating a thermal oxide film formed in the steps; forming an active layer of a thin film transistor by implementing patterning; and forming a thermal oxide film which composes at least a part of a gate insulating film on the surface of the active layer by means of thermal oxidation.
(4) The present invention provides a method for fabricating a semiconductor device, comprising steps of selectively introducing a metal element which promotes crystallization of silicon to an amorphous silicon film; growing crystal by a first heat treatment in a direction parallel to the film from the domain to which the metal element has been selectively introduced; forming a thermal oxide film on the surface of the domain where the crystal has been grown by implementing a second heat treatment within an oxidizing atmosphere; eliminating the thermal oxide film; and forming an active layer of the semiconductor device by using the domain from which the thermal oxide film has been eliminated.
(5) The present invention provides a semiconductor device, characterized in that the semiconductor device has a crystal silicon film interposed between first and second oxide films; the crystal silicon film contains a metal element which promotes crystallization of silicon; and the metal element is distributed in high concentration near the interfaces with the first and/or second oxide film within the crystal silicon film.
(6) The present invention provides a semiconductor device comprising an underlying layer made from an oxide film; a crystal silicon film formed on the underlying layer; and a thermal oxide film formed on the crystal silicon film; wherein the crystal silicon film contains a metal element which promotes crystallization of silicon; the metal element which promotes the crystallization of silicon is distributed in high concentration near the interface with the underlying layer and/or the thermal oxide film; and the thermal oxide film composes at least a part of a gate insulating film of a thin film transistor.
(7) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(8) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to form a thermal oxide film on the surface of the crystal silicon film and by causing the thermal oxide film to getter the metal element; eliminating the thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(9) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element; eliminating a thermal oxide film formed in the previous step; forming an active layer of a thin film transistor by implementing patterning; and forming another thermal oxide film which composes at least a part of a gate insulating film on the surface of the active layer by means of thermal oxidation.
(10) The present invention provides a method for fabricating a semiconductor device, comprising steps of selectively introducing a metal element which promotes crystallization of silicon to an amorphous silicon film growing crystal by a first heat treatment in a direction parallel to the film from the domain to which the metal element has been selectively introduced; forming a thermal oxide film on the surface of the domain where the crystal has been grown by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element; eliminating the thermal oxide film; and forming an active layer of the semiconductor device by using the domain from which the thermal oxide film has been eliminated.
(11) The present invention provides a semiconductor device, characterized in that the semiconductor device has a crystal silicon film interposed between a first and second oxide films; the crystal silicon film contains hydrogen and a halogen element as well as a metal element which promotes crystallization of silicon; and the metal element is distributed in high concentration near the interfaces with the first and/or second oxide film within the crystal silicon film.
(12) The present invention provides a semiconductor device, comprising an underlying layer made from an oxide film; a crystal silicon film formed on the underlying layer; and a thermal oxide film formed on the crystal silicon film; wherein the crystal silicon film contains a metal element which promotes crystallization of silicon, hydrogen and a halogen element; the metal element which promotes the crystallization of silicon is distributed in high concentration near the interface with the underlying layer and/or the thermal oxide film; the halogen element is distributed in high concentration near the interface with the underlying layer and/or the thermal oxide film; and the thermal oxide film composes at least part of a gate insulating film of a chin film transistor
(13) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; irradiating laser light or intense light to the crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(14) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; irradiating laser light or intense light to the crystal silicon film to diffuse the metal element, existing within the crystal silicon film, in the crystal silicon film; implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to cause the metal element existing within the crystal silicon film to be gettered to a thermal oxide film to be formed; eliminating the thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(15) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally and selectively introducing a metal element which promotes crystallization of silicon to an amorphous silicon film; implementing a first heat treatment to the amorphous silicon film to grow crystal in a direction parallel to the film from a domain of the amorphous silicon film into which the metal element has been intentionally and selectively introduced; irradiating laser light or intense light to diffuse the metal element existing within the domain where the crystal has grown; implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to cause the metal element existing within the domain where the crystal has grown to be gettered to a thermal oxide film to be formed; eliminating the thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(16) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; forming an active layer of the semiconductor device by patterning the crystal silicon film; irradiating laser light or intense light to the active layer; implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to eliminate or reduce the metal element existing within the active layer; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the active layer by implementing another thermal oxidation.
(17) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; forming an active layer of the semiconductor device by patterning the crystal silicon film; irradiating laser light or intense light to the active layer; implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to eliminate or reduce the metal element existing within the active layer; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the active layer by implementing another thermal oxidation, wherein the active layer has an inclined shape in which an angle formed between a side face and an underlying face is 20xc2x0 to 50xc2x0.
(18) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; irradiating laser light or intense light to the crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(19) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; irradiating laser light or intense light to the crystal silicon film to diffuse the metal element, existing within the crystal silicon film, in the crystal silicon film; implementing a second heat treatment within an oxidizing atmosphere to cause the metal element existing within the crystal silicon film to be gettered to a thermal oxide film to be formed; eliminating the thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(20) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally and selectively introducing a metal element which promotes crystallization of silicon to an amorphous silicon film; implementing a first heat treatment to the amorphous silicon film to grow crystal in a direction parallel to the film from a domain of the amorphous silicon film into which the metal element has been intentionally and selectively introduced; irradiating laser light or intense light to diffuse the metal element existing within the domain where the crystal has grown; implementing a second heat treatment within an oxidizing atmosphere to cause the metal element existing within the domain where the crystal has grown to be gettered to a thermal oxide film to be formed; eliminating the thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
(21) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; forming an active layer of the semiconductor device by patterning the crystal silicon film; irradiating laser light or intense light to the active layer; implementing a second heat treatment within an oxidizing atmosphere to eliminate or reduce the metal element existing within the active layer; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the active layer by implementing another thermal oxidation.
(22) The present invention provides a method for fabricating a semiconductor device, comprising steps of intentionally introducing a metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; forming an active layer of the semiconductor device by patterning the crystal silicon film; irradiating laser light or intense light to the active layer; implementing a second heat treatment within an oxidizing atmosphere to eliminate or reduce the metal element existing within the active layer; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film on the surface of the active layer by implementing another thermal oxidation, wherein the active layer has an inclined shape in which an angle formed between a side face and an underlying face is 20xc2x0 to 50xc2x0.
(23) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film on a substrate having an insulating surface; intentionally introducing a metal element which promotes crystallization of silicon to the amorphous silicon film; obtaining a crystal silicon film by crystallizing the amorphous silicon film by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C.; forming an active layer of the semiconductor device by patterning the crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element; eliminating a thermal oxide film formed in the previous step; and forming another thermal oxide film after eliminating the thermal oxide film by implementing another thermal oxidation; wherein a temperature of the second heat treatment is higher than that of the first heat treatment.
(24) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film on a substrate having an insulating surface; intentionally introducing a metal element which promotes crystallization of silicon to the amorphous silicon film; obtaining a crystal silicon film by crystallizing the amorphous silicon film by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C.; forming an active layer of the semiconductor device by patterning the crystal silicon film; implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to cause the metal element existing within the crystal silicon film to be gettered to a thermal oxide film to be formed; eliminating the thermal oxide film formed in the previous step; and forming another thermal oxide film after eliminating the thermal oxide film by implementing another thermal oxidation; wherein a temperature of the second heat treatment is higher than that of the first heat treatment.
(25) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film on a substrate having an insulating surface; intentionally and selectively introducing a metal element which promotes crystallization of silicon to the amorphous silicon film; growing crystal in a direction parallel to the film from a domain of the amorphous silicon film into which the metal element has been intentionally and selectively introduced by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C.; forming an active layer of the semiconductor device by using the domain in which the crystal has been grown in the direction parallel to the film by patterning; implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to cause the metal element existing within the active layer to be gettered to a thermal oxide film to be formed; eliminating the thermal oxide film formed in the previous step; and forming another thermal oxide film after eliminating the thermal oxide film by implementing another thermal oxidation; wherein a temperature of the second heat treatment is higher than that of the first heat treatment.
(26) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film; holding a metal element which promotes crystallization of silicon in contact on the surface of the amorphous silicon film; crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; forming a thermal oxide film on the surface of the crystal silicon film by implementing a second heat treatment in the temperature range of 500xc2x0 C. to 700xc2x0 C. within an atmosphere containing oxygen, hydrogen and fluorine; and eliminating the thermal oxide film.
(27) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film; holding a metal element which promotes crystallization of silicon in contact on the surface of the amorphous silicon film; crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; forming a thermal oxide film on the surface of the crystal silicon film by implementing a second heat treatment in the temperature range of 500xc2x0 C. to 700xc2x0 C. within an atmosphere containing oxygen, hydrogen, fluorine and chlorine; and eliminating the thermal oxide film.
(28) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film; holding a metal element which promotes crystallization of silicon in contact on the surface of the amorphous silicon film; obtaining a crystal silicon film by crystallizing the amorphous silicon film by a heat treatment; forming a wet oxide film on the surface of the crystal silicon film within an atmosphere containing fluorine/chlorine; and eliminating the oxide film.
(29) The present invention provides a semiconductor device having a silicon film having a crystallinity, characterized in that the silicon film contains a metal element which promotes crystallization of silicon in concentration of 1xc3x971016 cmxe2x88x923 to 5xc3x971018 cmxe2x88x923, fluorine atoms in concentration of 1xc3x971015 cmxe2x88x923 to 1xc3x971020 cmxe2x88x923, and hydrogen atoms in concentration of 1xc3x971017 cmxe2x88x923 to 1xc3x971021 cmxe2x88x923. It is noted that the unit of concentration xe2x80x9c . . . cmxe2x88x923xe2x80x9d means the number of atoms (atoms/cm3) per 1 cc and the same applies throughout the present specification.
(30) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film; crystallizing the amorphous silicon film to form a crystal silicon film; growing a thermal oxide film on the surface of the crystal silicon film by heating in an oxidizing atmosphere to which fluorine compound gas is added; eliminating the thermal oxide film on the surface of the crystal silicon film; and depositing an insulating film on the surface of the crystal silicon film.
(31) The present invention provides a method for fabricating a semiconductor device, comprising steps of forming an amorphous silicon film; irradiating laser light to crystallize the amorphous silicon film to form a crystal silicon film; growing a thermal oxide film on the surface of the crystal silicon film by heating in an oxidizing atmosphere to which fluorine compound gas is added; eliminating the thermal oxide film on the surface of the crystal silicon film; and depositing an insulating film on the surface of the crystal silicon film.
(32) The present invention provides a method for fabricating a semiconductor device in fabricating a thin film transistor on a substrate having an insulating surface, comprising steps of forming an amorphous silicon film; crystallizing the amorphous silicon film to form a crystal silicon film; growing a thermal oxide film on the surface of the crystal silicon film by heating in an oxidizing atmosphere to which fluorine compound gas is added; eliminating the thermal oxide film on the surface of the crystal silicon film; forming an active layer of the thin film transistor by shaping the crystal silicon film; depositing an insulating film on the surface of the active layer to form a gate insulating film at least on the surface of a channel region; forming a gate electrode on the surface of the gate insulating film; and forming a source and a drain in a manner of self-alignment by injecting impurity ions which give a conductive type to the active layer by using the gate electrode as a mask.
(33) The present invention provides a method for fabricating a semiconductor device in fabricating a thin film transistor on a substrate having an insulating surface, comprising steps of forming an amorphous silicon film; forming a crystal silicon film by crystallizing the amorphous silicon film; irradiating laser light to the crystal silicon film; growing a thermal oxide film on the surface of the crystal silicon film by heating in an oxidizing atmosphere to which fluorine compound gas is added; eliminating the thermal oxide film on the surface of the crystal silicon film; forming an active layer of the thin film transistor by shaping the crystal silicon film; depositing an insulating film on the surface of the active layer to form a gate insulating film at least on the surface of a channel region; forming a gate electrode on the surface of the gate insulating film; and forming a source and a drain in a manner of self-alignment by injecting impurity ions which give a conductive type to the active layer by using the gate electrode as a mask.
According to one typical aspect of the present invention, a metal element which promotes crystallization of silicon is introduced to the surface of an amorphous silicon film formed in advance to form a crystal silicon film. Next, a thermal oxide film is formed on the surface of the crystal silicon film to cause the metal element to move or to be gettered to the thermal oxide film to reduce the concentration of the metal element or to eliminate the metal element within the crystal silicon film.
The amorphous silicon film may be formed by means of normal methods such as plasma CVD. The amorphous silicon film is formed on a surface of adequate solid body or on a substrate when it is used in constructing a semiconductor device. For the substrate, a ceramic substrate or the like, beside a glass substrate and a quartz substrate, may be used. While the amorphous silicon film is formed also on a film such as a silicon oxide film formed on the surface of such substrate, the substrate which is referred in the present specification means to include such aspects.
Next, the metal element which promotes the crystallization of silicon is introduced to the surface of the amorphous silicon film formed in advance as described above. As the metal element which promotes the crystallization of silicon, one or a plurality of types of metal elements selected from iron (Fe), nickel (Ni), cobalt (Co), ruthenium (Ru), rhodium (Rh), paradium (Pd), osnium (Os), iridium (Ir), platinum (Pt), copper (Cu), and gold (Au) is used. These metal elements are used as the metal elements which promote the crystallization of silicon in any of the inventions described in the present specification and are referred in the present specification as xe2x80x9cmetal elements which promote the crystallization of silicon typified by nickelxe2x80x9d as necessary.
While these metal elements may be introduced: 1) on the whole surface of the amorphous silicon film, 2) at end portions of the amorphous silicon film (if the face of the amorphous silicon film is rectangular, at the end of one side, ends of two sides, ends of three sides or ends of four sides: if the face of the amorphous silicon film is circular, at its peripheral portion), 3) to the center of the face of the amorphous silicon film, 4) in dots (that is, in dots leaving predetermined spaces therebetween on the surface of the amorphous silicon film) and the like and there is no specific limitation, it is preferred to introduce on the whole surface or to the end portions of the amorphous silicon film. Further, although it is possible to adopt an aspect of introducing the metal element on the back face of the amorphous silicon film, it is preferable to apply it on the front surface from the aspect of fabrication of the semiconductor device.
Further, there is no specific limit on the method how to introduce those metal elements to the amorphous silicon film so long as it is a method which allows the metal elements to be introduced to the surface or the inside of the amorphous silicon film, and such methods as sputtering, CVD, plasma treatment (including plasma CVD), adsorption and a method of applying solution of metallic salt may be used for example. Among them, the method of using solution is useful from the aspects that it is simple and that the concentration of the metal element may be readily adjusted. Various salts may be used for the metallic salt and organic solvents such as alcoholic, aldehyde, ether solvents or a mixed solvent of water and organic solvent may be used beside water as the solvent. Further, the solution needs not be what such metallic salt is dissolved completely and may be what part or whole of the metallic salt exists in suspended state.
Any type of the metallic salt can be used regardless whether it is organic salt or non-organic salt so long as it is salt which can exists as the solution or suspended solution as described above. For example, such ferrous salt as ferrous bromide, ferric bromide, ferric acetate, ferrous chloride, ferric chloride, ferric fluoride chloride, ferric nitrate, ferrous phosphate, ferric phosphate and the like and such cobalt salt as cobalt bromide, cobalt acetate, cobalt chloride, cobalt fluoride, cobalt nitrate and the like may be used.
Further, such nickel salt as nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel oxide, nickel hydroxide, nickel acetylacetate, nickel 4-cyclohexylbutyrate, nickel etylhexanoic acid and the like may be used. Ruthenium chloride may be cited as an example of ruthenium salt, rhodium chloride as an example of rhodium salt, paradium chloride as an example of paradium salt, osnium chloride as an example of osmium salt, iridium trichloride or iridium tetrachloride as examples of iridium salt, platinic chloride as an example of platinum salt, cupric acetate, cupric chloride and cupric nitrate as examples of copper salt, and gold trichloride and gold chloride as gold salt.
After thus introducing the metal element to the amorphous silicon film, the crystal silicon film is formed by using the metal element. While this crystallization may be carried out by implementing a heat treatment (Solid Phase Crystallization) or by irradiating laser light or intense light such as ultraviolet ray or infrared ray, it is preferable to use the heat treatment. While this solid phase crystallization proceeds even in an atmosphere containing hydrogen or oxygen, preferably an inactive atmosphere such as nitrogen or argon is used. It is noted chat this heat treatment or the heat treatment temperature will be referred to as the xe2x80x9cfirst heat treatmentxe2x80x9d or xe2x80x9ctemperature of the first heat treatmentxe2x80x9d as necessary throughout the present specification.
The first heat treatment may be carried out in the temperature range of 400 to 1100xc2x0 C. or preferably about 550 to 1050xc2x0 C. Although the crystallization proceeds even at a temperature of about 400xc2x0 C., the crystallization speed is slow and it takes a long time in this case. Accordingly, the temperature is preferable to be above 550xc2x0 C. or more preferably above 700xc2x0 C. The higher heat treatment temperature allows a better quality crystal to be obtained and the crystallization speed to be increased.
While the first heating temperature is limited to about 600 to 650xc2x0 C. from the aspect of distortion point when a glass substrate whose distortion point is 667xc2x0 C. for example is used as the substrate, it is needless to say that the temperature may be increased further if a glass substrate having a high heat resistance is used. While a temperature of about 1100xc2x0 C. may be applied when the substrate is a quartz substrate, it is preferred to be less than about 1050xc2x0 C. Further, the irradiation of laser light or intense light may be carried out after the heat treatment.
Then, the thermal oxide film is formed on the surface of the crystal silicon film. Thereby, the concentration of the metal element within the crystal silicon film may be reduced or the metal element may be eliminated by causing the metal element to move into or to be gettered to the thermal oxide film according to the present invention. While an oxidizing atmosphere is used in forming the thermal oxide film, it is preferably 1) an oxygen atmosphere, 2) an atmosphere containing oxygen, 3) an atmosphere containing a compound which releases oxygen at the temperature in forming the thermal oxide film, or 4) an atmosphere containing oxygen in 1), 2) and 3) above and halogen.
The thermal oxide film may be formed in the same temperature range with that of the solid phase crystallization described above, i.e. in the range of about 400 to 1100xc2x0 C., or preferably about 700 to 1050xc2x0 C. While this temperature may be about the same with that applied to the solid phase crystallization (temperature of the first heat treatment), it is preferable to be higher than that applied to the solid phase crystallization. Thereby, the thermal oxide film may be formed and the solid phase crystallization may be advanced further as compared to the case when the same temperature with that of the first heat treatment is applied.
While the thermal oxide film is thus formed on the surface of the crystal silicon film, the effect of oxygen or oxygen and halogen within the oxidizing atmosphere causes the metal element to be gettered to the thermal oxide film and the concentration of the metal element within the crystal silicon film is reduced or the metal element is eliminated. It is noted that the heat treatment for forming the thermal oxide film and its temperature are referred to xe2x80x9csecond heat treatmentxe2x80x9d and xe2x80x9ctemperature of the second heat treatmentxe2x80x9d as necessary throughout the present specification.
Next, the thermal oxide film which has gettered the metal element is eliminated. Although there is no limit on the method for eliminating the thermal oxide film so long as it is a method which can eliminate the thermal oxide film, it may be carried out by using a hydrofluoric acid type etchant such as buffer hydrofluoric acid. Thus, the crystal silicon film having a high crystallinity and from which the metal element has been eliminated or the concentration of the metal element has been reduced can be obtained. This crystal silicon film has excellent characteristics as an element (e.g. active layer) within a semiconductor device.
FIGS. 1 through 4 show microphotographs of several examples of the crystal silicon film. Among the figures, FIG. 1 shows a case in which nickel element has been applied to one end of a rectangular amorphous silicon film to crystallize it and FIG. 2 shows a case in which nickel element has been applied to the whole surface of an amorphous silicon film to crystallize it. As it is apparent from FIG. 1, the crystal has grown from one end to the other end in parallel or almost in parallel. In the example in FIG. 2 in which nickel element has been applied to the whole surface of the amorphous silicon film to grow crystal, star-like light and shades can be seen and it can be seen that crystals have grown radially centering on a number of points.
FIGS. 3 and 4 are photographs taken by a transmission type electron microscope. The crystal silicon film shown in the photographs is what has been obtained approximately through the process of (A) through (G) in FIGS. 5A through 5G which show the process diagrammatically (the process similar to those in embodiments described later).
(A) A quartz substrate having a fully smooth surface is cleaned and an amorphous silicon film is formed thereon in a thickness of 500 angstrom by means of low pressure thermal CVD (LPCVD).
(B) Next, a silicon oxide film is formed in a thickness of 700 angstrom by means of CVD using TEOS (tetraethoxisilane) and an opening is formed by patterning it. The amorphous silicon is exposed at the bottom of the opening.
(C) Nickel acetate solution containing nickel in concentration (weight conversion) of 100 ppm is applied as shown in FIG. 5C by a spin-coater.
(D) A heat treatment is implemented within a nitrogen atmosphere at 600xc2x0 C. for eight hours in the state while adhering the nickel acetate solution.
(E) A mask of the silicon oxide film is removed to obtain a crystal silicon film having a region where crystal has grown laterally.
(F) A heat treatment is implemented within an oxygen atmosphere (atmospheric pressure) containing 3 volume % of HCl at 950xc2x0 C. for 20 minutes. As a result, an oxide film of 200 angstrom is formed and the thickness of the silicon film is reduced to 400 angstrom. It is noted that while the reason why the thickness of the crystal silicon film is reduced is unknown in detail and we would have to wait for the study of the future, it is assumed-to have happened because silicon in non-crystallized state or not crystallized completely is consumed in the formation of the thermal oxide film.
(G) The oxide film formed in step (F) is eliminated by using buffer hydrofluoric acid.
As it is apparent from FIGS. 3 and 4, the crystal in the crystal silicon film has grown 1) such that a structure of crystal lattices lies continuously in a row, 2) so as to be thin cylindrical crystal or thin flat cylindrical crystal, and 3) so as to be a plurality of thin cylindrical crystals or thin flat cylindrical crystals in parallel or almost in parallel leaving a space therebetween. Further, seeing the photograph of FIG. 4, it can be seen that a cylindrical crystal of 0.15 xcexcm in width extends diagonally from the lower right corner to the upper left corner and that there is a clear boundary (grain boundary) at the edges of the both ends.
FIGS. 7a and 7b show the form of crystal growth in the crystal silicon film obtained by the present invention and assumed from the result observed from a number of photographs of the electron microscope typified by FIGS. 1 through 4. FIG. 7a shows one exemplary case when the crystal has been grown by introducing the metal element which promotes the crystallization of silicon into one end of the surface of the amorphous silicon film. In this case, the crystals of silicon grow linearly from the region where the metal has been added in parallel or almost in parallel.
FIG. 7b shows a case when the crystal has been grown by applying the metal element which promotes the crystallization of silicon on the whole surface of the amorphous silicon film. In this case, the crystals of silicon grow radially centering from a numerous points on the whole surface of the amorphous silicon film. Seeing from the mutual relationship of position of adjacent radial crystal cylinders which extend centering from respective points, each crystal grows linearly and in parallel or almost in parallel.
By the way, it is effective to shorten a channel length in order to increase an operating speed of a TFT for example (while the same applies to a MOS type transistor in general, this point will be described centering on TFT here) However, if the channel length is shortened below 1 xcexcm for example, a trouble called a short-channel effect is brought about. In concrete, problems such as the degradation of sub-threshold characteristic and the decrease of threshold value occur.
Here, the sub-threshold characteristic (referred also as S value) means a build-up characteristic when a switch of the TFT is turned on as shown diagrammatically in FIG. 8. In concrete, if the build-up is sharp, the sub-threshold characteristic is good and the TFT may be operated at high speed. On the other hand, a TFT having a bad sub-threshold characteristic has a build-up curve whose inclination is small (i.e. the curve is lying) and it is not suited to high-speed operation.
The degradation of the sub-threshold characteristic in the short-channel effect may be explained from the present technological knowledge (=present technological knowledge or prior art theory) as follows. Firstly, what the channel is shortened means that a distance between a source region and a drain region is shortened. Generally, a channel is intrinsic (I type semiconductor) and a source/drain region is N- or P-type semiconductor. If an intrinsic semiconductor contacts with an N-type semiconductor for example, the quality of the N-type semiconductor as the semiconductor exerts influence to the inside of the intrinsic semiconductor.
In the case of the TFT, the above-mentioned influence is exerted to the inside of the channel. That is, the influence of N-type or P-type is exerted from the source region or drain region to the inside of the channel. The degree of this influence, i.e. a range in which the influence is exerted, does not change even if the channel is shortened.
If the channel length is shortened further, the influence exerted from the source/drain region to the channel with. respect to the size of the channel length becomes significant. In an extreme case, the range of the influence exerted from the source/drain region to the inside of the channel may become longer than the channel length. in such a state, a trouble occurs in the operation of the TFT (the same applies also to MOS type transistors) that the change of conductive type of the channel is controlled by the application of electric field from a gate electrode and electric conductivity between the source and the drain changes, thus degrading the sub-threshold characteristic as a result.
A TFT using the crystal silicon film obtained by the present invention has a channel length of less than about 1 xcexcm. Accordingly, it is presumed that the short-channel effect appears naturally from the technological knowledge as described above.
However, it has. been found that in the crystal of the crystal silicon film obtained by the present invention, i.e. the crystal which have grown 1) such that a structure of crystal lattices lies continuously in a row, 2) so as to be thin cylindrical crystal or thin flat cylindrical crystal, and 3) so as to be a plurality of thin cylindrical crystals or thin flat cylindrical crystals in parallel or almost in parallel leaving a space therebetween, not only no short-channel effect is seen, but also a very good sub-threshold characteristic which cannot be explained by the prior art technological knowledge is seen and that it operates at high speed corresponding to such characteristic.
Tables 1 and 2 and FIG. 9 show one example thereof. The semiconductor device used here is what has been fabricated in the process of (H) through (L) in FIGS. 6H through 6L below which continue from the process shown in FIGS. 5A through 5G described above. It is noted that Step G in FIG. 6G corresponds to Step G in the process shown in FIG. 5G.
(H) The crystal silicon film formed in the process from (A) through (F) is patterned to form an active layer of a thin film transistor.
(I) Next, a silicon oxide film is formed as a GI film (gate insulating film) by using mixed gas of SiH4+N2O as film forming gas by means of plasma CVD.
(J) A heat treatment is implemented in an oxygen atmosphere (atmospheric pressure) containing 3 volume % of HCl at 950xc2x0 C. for 28 minutes. As a result, a thermal oxide film of 300 angstrom in thickness is formed and the thickness. of the crystal silicon film is reduced to 250 angstrom. It is noted that while the reason why the thickness of the crystal silicon film is reduced is unknown similarly to the aforementioned case in forming the thermal oxide film and we would have to wait for the research of the future, it is assumed to have happened because silicon in non-crystallized state or not crystallized completely is consumed in the formation of the thermal oxide film. It is also noted here that the thermal oxide film is formed on the surface of the active layer in connection with the fact that activated oxygen molecules infiltrate into the GI film.
(K) An aluminum film of 4000 angstrom thickness is formed by sputtering. It is noted that 0.18 weight % of scandium is contained in the aluminum. Then, an anodic oxide film is formed further on the surface of the aluminum film.
(L) Next, a resist mask is placed and the aluminum film is patterned to fabricate a prototype of a gate electrode.
Table 1 shows characteristics of an N-channel type TFT and Table 2 shows that of a P-channel. type TFT constructed by using the crystal silicon film of the present invention. In Tables 1 and 2, measurement points 1 through 20 mean that they are measured by using each spot on the surface of one batch of the crystal silicon film fabricated as described above. As it is apparent from Table 1, when the crystal silicon film is constructed as the N-channel type TFT, S-value is very small in particular among the characteristics. it is around 80 mV/decade and is within a range of 70 to 90 mV/decade as a whole. It is so small as 72.53 mV/decade especially at the measurement point 13.
The S-value (sub-threshold coefficient) is defined as an inverse number of a maximum inclination at the build-up portion of the curve of ID-VG as shown in FIG. 8. In other words, it is understood as an increment of gate voltage necessary for increasing drain current by one digit. That is, the smaller the S-value, the sharper the inclination at the build-up portion. Then, it excels in responsibility as a switching element and can be operated at high speed.
An ideal value derived from a theoretical formula is 60 mV/decade. Although a value close to that is obtained in a transistor using a mono-crystal wafer, a conventional TFT using low temperature poly-silicon is limited to 300 to 500 mV/decade. In view of this fact, the S-value of around 80 mV/decade of the TFT using the crystal silicon film of the present invention can be said as an astonishing value.
In Table 2 showing the P-channel type TFT constructed by using the crystal silicon film of the present invention, the S-value is also very small in this case similarly to the case of the N-channel type TFT. It is around 80 mV/decade and is within a range of 70 to 100 mV/decade as a whole. It is so small as 72.41 mV/decade especially at the measurement point 4. These values mean that it is the same with the case of the N-channel type TFT, except only of that plus (+) and minus (xe2x88x92) are opposite.
Beside the above, each of the characteristics (codes or signs) means as follows. As it is apparent from Tables 1 and 2, any of these characteristics shows values which can fully sustain in practical use. Ion is drain current which flows when the TFT is ON and it is set as Ionxe2x88x921 when VD=1 V (1 volt) and Ionxe2x88x922 when VD=5V. A TFT having a larger Ion value can flow more current in a short time.
Ioff is drain current which flows when the TFT is OFF and it is set as Ioffxe2x88x921 when VD=1 V (1 volt) and Ioffxe2x88x922 when VD=5V. If current flows when the TFT is OFF, electric power is consumed that much, so that it is very important to minimize Ioff. If Ioff is large, there arises a problem that charge held in a liquid crystal flows out by Ioff. Ion/Ioffxe2x88x921 (or Ion/Ioffxe2x88x922) is a ratio between Ionxe2x88x921 and Ioffxe2x88x921 and represents how many digits the ON current differs from the OFF current. The greater the Ion/Ioff, the better the switching characteristic is. It is important also in increasing contrast on a display panel.
Vth is a parameter generally called as a threshold voltage and is defined as voltage when a TFT is switched to ON for example. Values within the table are those obtained by means of root ID extrapolation by setting as the objects of evaluation when VD=5. If Vth is large, voltage applied to a gate electrode has to be set high, so that driving voltage as well as power consumption increase. xcexcFE represents a mobility of field effect. It is a parameter indicating the mobility of carriers. A TFT having a large xcexcFE can be said to be suitable for high-speed operation. As it is apparent from Tables 1 and 2, any of these characteristics show values which can fully sustain in practical use.
FIGS. 9a and 9b are graphs of the IDxe2x88x92VG characteristic drawn by selecting typical values from the above-mentioned actually measured data. FIG. 9a shows the case of the N-channel type TFT and FIG. 9b shows the case of the P-channel type TFT. In the both cases, VD=1V. The horizontal axis within FIGS. 9a and 9b represents gate voltage (V) and the vertical axis represents drain current (A). The scale unit of the vertical axis is xe2x80x9c1E-13xe2x80x9d through xe2x80x9c1E-01xe2x80x9d, i.e. within a range of 1xc3x9710xe2x88x9213 through 1xc3x9710xe2x88x921 A (Ampere).
Seeing the case of the N-channel type TFT in FIG. 9a at first, it can be seen that the inclination of the build-up portion of the ID-VG curve, i.e. the curve at the linear region, is very sharp. It shows the characteristic corresponding to that the S-value described above is small as it is and indicates that the TFT has an excellent responsibility as a switching element and can be operated at high speed. Further, while the range between xe2x88x926 V to xe2x88x920.5 V of gate voltage in FIG. 9a corresponds to Ioff within Table 1 described above, it can be seen that the drain current flowing when the TFT is OFF is very small and that the TFT has another excellent quality also in this aspect.
Next, seeing FIG. 9b, the curve at the linear region is very sharp and the drain current flowing when the TFT is OFF is very small also in the case of the P-channel type TFT. It also has an excellent characteristics similarly to the N-channel type TFT described above. It is noted that as for such technological significance, only the codes (signs 9 of plus (+) and minus (xe2x88x92) are different as compared to the case of the N-channel type TFT.
FIGS. 11a and 11b show oscillograms obtained by constructing a ring oscillator by building a circuit in which the N-channel type TFT and the P-channel type TFT described above are combined and by operating it. This circuit works such that the N-channel type TFT and the P-channel type TFT compensate their operation each other in the same time, i.e. such that when one TFT discharges electric charge, the other TFT sucks the electric charge.
FIG. 10 is a diagrammatic view for explaining FIGS. 11a and 11b. When FIG. 10 is seen as a whole, the waveform on the + side of the oscillating waveform is related mainly with the operation of the N-channel type TFT and the waveform on the xe2x88x92 side is related mainly with the operation of the P-channel type TFT. Accordingly, when it )is oscillated in 152.0 MHz, 252.9 MHz or the like and if the oscillating waveform keeps symmetry on the + and xe2x88x92 sides, it means that the N-channel type TFT and the P-channel type TFT operate symmetrically with respect to the frequency and operate normally with the similar characteristics.
Then, seeing the oscillograms in FIGS. 11a and 11b, the oscillating waveform is a sine wave, no distortion is seen in the linearity and they are symmetrical both vertically and horizontally. Thus, it can be seen that the inventive crystal silicon film shows the excellent characteristics even when it is applied as the N-channel type or P-channel type and that substantially there is no difference in the characteristics between the both.
A model which can explain the above-mentioned phenomena and characteristics may be considered as follows. At first, as seen in the photographs of the electron microscope shown in FIGS. 3 and 4, the silicon semiconductor thin film composing the TFT composed of the crystal silicon film obtained by the present invention has a structure in which the crystal continues in the specific direction. According to detailed observation by means of the electron microscope, it has been confirmed that the lattice structures are continuing in the specific direction.
From the above-mentioned observation result, this state is construed as a state in which mono-crystals are continuing in the specific direction leaving a predetermined space therebetween. It is then understood, naturally, that carries can readily move in the direction to which the lattice structures continue. That. is, it is assumed that the channel region is composed of countless thin and long channels in the TFT using the crystal silicon film obtained by the present invention.
Here, while linear grain boundaries observed in the photographs in FIGS. 3 and 4 partition very small channels, no state in which impurities segregate specifically at the grain boundary is seen.
Such grain boundary contains mismatch and distortion of crystal structures and an energy level there is considered to be higher than the other regions. Accordingly, it is assumed to have a function of restricting the move of the carriers in the direction to which the crystal structures are continuing. If such narrow and small channels are formed, a range of osmosis of influence from the source and drain regions exerted to the inside of the small channels are considered to become small corresponding to the narrowness.
As it is analogized how electromagnetic wave expands in a space where there is no obstacle for example, the electrical influence is assumed to expand, ideally, isotropically in two- or three-dimension. In view of such fact, because the countless small and narrow channels are formed in the TFT using the crystal silicon film obtained by the present invention, it can be understood that the influence from the source and drain regions to the channel is suppressed in each individual small channel and that it suppresses the short-channel effect as a whole.
FIGS. 12 and 13 are graphs of measured values of gate current of a planar type thin film transistor which the inventors et. al. have made in trial by using the crystal silicon film in which the concentration of the metal element which promotes the crystallization of silicon is reduced in the processes of numerous studies and tests conducted until reaching to the present invention. FIG. 12 is different from FIG. 13 in that whether thermal oxidation is used or plasma CVD is used in forming a gate insulating film.
That is, FIG. 12 shows measured values obtained when the gate insulating film has been formed by the thermal oxidation and FIG. 13 shows measured values obtained when the gate insulating film has. been formed by the plasma CVD. in FIGS. 12 and 13, the horizontal axis represents the gate current and the vertical axis represents a number of measured samples. A quartz substrate is used as the substrate here. Further, an active layer is formed by holding nickel element in contact on the surface of the amorphous silicon film and by crystallizing by a heat treatment of four hours at 640xc2x0 C. The thermal oxide film is formed within an oxygen atmosphere at 950xc2x0 C.
It can be seen from FIG. 12 that the values of gate current vary largely depending on the samples. It shows that there is dispersion in the quality of the gate insulating film. Meanwhile, as shown in FIG. 13, there is less dispersion of the gate currents and its value is extremely small in the thin film transistor in which the gate insulating film has been formed by the plasma CVD. The reason why the difference of the measured values shown in FIGS. 12 and 13 appears may be explained as follows.
That is, nickel element is sucked from the active layer to the thermal oxide film during when the thermal oxide film is formed in the samples in which the gate insulating film is formed by the thermal oxide film. As a result, nickel element which hampers the insulation comes to exist within the thermal oxide film. The existence of the nickel element increases a value of current leaked within the gate insulating film and varies that value.
This fact is supported by SIMS (secondary ion mass spectrometry) and by measuring the concentration of nickel element within the gate insulating film of the samples whose measured values have been obtained in FIGS. 12 and 13. That is, it was confirmed that while nickel element of more than the level of 1017 cmxe2x88x923 is measured within the gate insulating film formed by the thermal oxidation, the concentration of nickel element within the gate insulating film formed by the plasma CVD is less than the level of 1016 cmxe2x88x923. It is noted that the concentration of impurity described in the present specification is defined as the minimum value of the measured values measured by the SIMS.
The point described above is one example of findings which the inventors et. al. have obtained in the process of numerous studies and experiments conducted until reaching to the present invention and the present invention is based on such findings. That is, the thermal oxide film is formed on the surface of the crystal silicon film obtained by utilizing the metal element which promotes the crystallization of silicon to getter the metal element within the thermal oxide film and to reduce the concentration of the metal element or to eliminate the metal element within the crystal silicon film as a result.
Regardless whether it is the amorphous silicon film or the crystal silicon film, the metal element is normally a harmful substance within a semiconductor device composed of a thin film transistor (TFT and the like) using the silicon film, so that the metal element needs to be eliminated from the silicon film as much as possible. According to the present invention, such metal element which promotes the crystallization of silicon may be eliminated or reduced very effectively after using it in the formation of the crystal silicon film. Thereby, the present invention allows a semiconductor device having such excellent characteristics to be obtained.
The main aspects of the present invention described above in (1) through (6) are as follows.
According to one aspect of the invention described above in (1) and (2), an amorphous silicon film is formed at first. Then, the amorphous silicon film is crystallized by the effect of the metal element typified by nickel which promotes the crystallization of silicon to obtain a crystal silicon film. This crystallization is carried out by heat treatment. In the state after the heat treatment, the metal element which has been intentionally introduced is contained in certain high concentration within the crystal silicon film.
Another heat treatment is implemented to the crystal silicon film in the above-mentioned state in the oxidizing atmosphere to form a thermal oxide film on the surface of the crystal silicon film. At this time, the metal element is gettered to the thermal oxide film, thus reducing the concentration of the metal element or eliminating the metal element within the crystal silicon film. Then, the thermal oxide film which has gettered the metal element is eliminated.
The crystal silicon film which has a high crystallinity and from which the metal element has been eliminated or in which the concentration of the metal element is low can be obtained through those steps. The thermal oxide film described above may be eliminated by using buffer hydrofluoric acid or other hydrofluoric etchant. This process for eliminating the thermal oxide film is implemented in the same manner in the process for eliminating the thermal oxide film in each aspect of the invention described below.
According to one aspect of the invention described above in (3) , an active layer of a thin film transistor is formed by implementing patterning after the above-mentioned steps. That is, this active layer is formed by the crystal silicon film from which the metal element has been eliminated or in which the concentration of the metal element is low. Then, a thermal oxide film which composes at least a part of a gate insulating film is formed on the surface of the active layer by means of thermal oxidation to compose a semiconductor device.
According to one aspect of the invention described above in (4), an amorphous silicon film is formed at first and a metal element which promotes crystallization of silicon is selectively introduced to the amorphous silicon film. There is no particular limit how to selectively introduce the metal element to the amorphous silicon film and various methods may be adopted, such as 1) introducing to one end portion of the amorphous silicon film, 2) introducing to one end portion of the amorphous silicon film leaving a space, and 3) introducing in dots on the whole surface of the amorphous silicon film leaving spaces therebetween. Thereby, the crystal is grown by a first heat treatment in a direction parallel to the film from the region to which the metal element has been selectively introduced.
Next, a thermal oxide film is formed on the surface of the region where the crystal has been grown by implementing a second heat treatment within an oxidizing atmosphere. At this time, the metal element is gettered to the thermal oxide film by the action of oxygen within the oxidizing atmosphere and the concentration of the metal element within the crystal silicon film is reduced or the metal element is eliminated therefrom. Further, the thermal oxide film is eliminated and an active layer of the semiconductor device is formed by using the region from which the thermal oxide film has been eliminated. FIG. 6 is a process flowchart schematically showing the main point of the arrangement described above.
In any of these aspects, the temperature of the second heat treatment is preferable to be higher than that of the first heat treatment and it is preferable to anneal in a plasma atmosphere containing oxygen and hydrogen after eliminating the thermal oxide film. Further, the concentration of oxygen contained in the amorphous silicon film is preferable to be 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923.
Thus, according to one aspect of the present invention, there is provided a semiconductor device having the crystal silicon film interposed between first and second oxide films and containing a metal element which promotes crystallization of silicon and which is distributed in high concentration near the interfaces with the first and/or second oxide film within the crystal silicon film. According to one aspect of this semiconductor device, the first oxide film is a silicon oxide film or silicon oxynitride film formed on a glass substrate or quartz substrate, the crystal silicon film composes an active layer of a thin film transistor and the second oxide film may be composed of a silicon oxide film or silicon oxynitride film which forms a gate insulating film.
Similarly to the case described above, according to one aspect of the present invention, there is provided a semiconductor device comprising an underlying layer made from an oxide film; a crystal silicon film formed on the underlying layer; and a thermal oxide film formed on the crystal silicon film; wherein the crystal silicon film contains a metal element which promotes crystallization of silicon; the metal element which promotes the crystallization of silicon is distributed in high concentration near the interface with the underlying layer and/or thermal oxide film; and the thermal oxide film composes at least a part of a gate insulating film of a thin film transistor.
The main aspects of the present invention described above in (7) through (12) are as follows.
According to one aspect of the invention described above in (7), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. The metal element is contained in the crystal silicon film in the state after the heat treatment. Then, the metal element existing within the crystal silicon film is eliminated or reduced by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element.
At this time, the. metal element is gettered to the thermal oxide film by the actions of oxygen, halogen and halogen and oxygen, thus reducing the concentration of the metal element within the crystal silicon film or eliminating the metal element. Then, after eliminating the thermal oxide film formed there, another thermal oxide film is formed on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
According to one aspect of the invention described above in (8), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, the metal element existing within the crystal silicon film is eliminated or reduced by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element to form a thermal oxide film on the surface of the crystal silicon film and by causing the thermal oxide film to getter the metal element. Then, after eliminating the thermal oxide film formed there, another thermal oxide film is formed on the surface of the domain from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
According to one aspect of the invention described above in (9), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization or silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, the metal element existing within the crystal silicon film is eliminated or reduced by implementing a second heat treatment within an oxidizing atmosphere containing a halogen element. After eliminating a thermal oxide film formed there, an active layer of a thin film transistor is formed by implementing patterning and another thermal oxide film which composes at least a part of a gate insulating film is formed on the surface of the active layer by means of thermal oxidation. FIG. 6 is a process flow chart schematically showing the main part of the arrangement described above.
According to one aspect of the invention described above in (10), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is selectively introduced to the amorphous silicon film. As modes for selectively introducing the metal element to the amorphous silicon film, various methods may be adopted, such as 1) introducing to one end portion of the amorphous silicon film, 2) introducing to one end portion of the amorphous silicon film leaving a space, and 3) introducing in dots on the whole surface of the amorphous silicon film leaving spaces therebetween. Thereby, the crystal is grown by a first heat treatment in a direction parallel to the film from the region to which the metal element has been selectively introduced.
Next, a thermal oxide film is formed on the surface of the region where the crystal has been grown by implementing a second heat treatment within an oxidizing atmosphere containing halogen element. The thermal oxide film is eliminated and an active layer of the semiconductor device is formed by using the region from which the thermal oxide film has been eliminated. This active layer is formed of the crystal silicon film from which the metal element has been eliminated or the crystal silicon film in which the concentration of the metal element is low.
In the methods for fabricating the semiconductor device described above, an atmosphere in which one or a plurality of types of gases selected from HCl, HF, HBr, Cl2, F2 and Br2 is added to O2 atmosphere may be used as the oxidizing atmosphere containing the halogen element.
Further, the temperature of the second heat treatment is preferable to be higher than that of the first heat treatment and it is preferable to anneal in the plasma atmosphere containing oxygen and hydrogen after eliminating the thermal oxide film. Further, the concentration of oxygen contained within the amorphous silicon film is preferable to be 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923.
By the fabrication method described above, there is provided the semiconductor device in (11), i.e. the semiconductor device having a crystal silicon film interposed between first and second oxide films, wherein the crystal silicon film contains hydrogen and a halogen element as well as a metal element which promotes crystallization of silicon and the metal element is distributed in high concentration near the interfaces with the first and/or second oxide film within the crystal silicon film.
In the semiconductor device, the halogen element is contained in high concentration in the first oxide film and/or near the interface between the first oxide film and the crystal silicon film and the halogen element is contained in high concentration near the interface in the crystal silicon film with the second oxide film. Further, the first oxide film is a silicon oxide film or silicon oxynitride film formed on a glass substrate or quartz substrate, the crystal silicon film composes an active layer of a thin film transistor and the second oxide film may be composed of a silicon oxide film or silicon oxynitride film which forms a gate insulating film.
Similarly to the case described above, according to one aspect of the present invention described above in (12) there is provided a semiconductor device comprising an underlying layer made from an oxide film; a crystal silicon film formed or the underlying layer; and a thermal oxide film formed on the crystal silicon film; wherein the crystal silicon film contains a metal element which promotes crystallization of silicon and hydrogen and halogen element; the metal element which promotes the crystallization of silicon is distributed in high concentration near the interface with the underlying layer and/or thermal oxide film; and the halogen element is distributed in high concentration near the interface with the underlying layer and/or thermal oxide film; and the thermal oxide film composes at least a part of a gate insulating film of a thin film transistor.
The main aspects of the present invention described above in (13) through (17) are as follows.
According to one aspect of the invention described above in (13), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. After that, laser light or intense light is irradiated to the crystal silicon film. Next, a second heat treatment is implemented in an oxidizing atmosphere containing a halogen element to eliminate or reduce the metal element existing within the crystal silicon film. Then, a thermal oxide film formed there is eliminated and another thermal oxide film is formed on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
According to one aspect of the invention described above in (14), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. After that, laser light or intense light is irradiated to the crystal silicon film to diffuse the metal element existing within the crystal silicon film.
Next, a second heat treatment is implemented in an oxidizing atmosphere containing a halogen element to getter the metal element existing within the crystal silicon film to a thermal oxide film to be formed. Then, the thermal oxide film formed there is eliminated and another thermal oxide film is formed on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
According to one aspect of the invention described above in (15), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film. As modes for selectively introducing the metal element to the amorphous silicon film, various methods may be adopted, such as 1) introducing to one end portion of the amorphous silicon film, 2) introducing to one end portion of the amorphous silicon film leaving a space, and 3) introducing in dots on the whole surface of the amorphous silicon film leaving spaces therebetween. Thereby, the crystal is grown by a first heat treatment described below in a direction parallel to the film from the region to which the metal element has been selectively introduced.
The first heat treatment is implemented to the amorphous silicon film to grow crystal in a direction parallel to the film from a region of the amorphous silicon film into which the metal element has been intentionally and selectively introduced. After that, laser light or intense light is irradiated to diffuse the metal element existing within the region where the crystal has grown. A second heat treatment is implemented within an oxidizing atmosphere containing a halogen element to cause the metal element existing within the region where the crystal has grown to be gettered to a thermal oxide film to be formed. Then, the thermal oxide film formed here is eliminated and another thermal oxide film is formed on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
In the aspects of the invention described above in (13) through (15), it is preferable to conduct the second heat treatment in temperature above 600xc2x0 C. and below 750xc2x0 C. and it is preferable to form the gate insulating film by using said another thermal oxide film. Further, in those aspects of the invention, the atmosphere in which one or a plurality of types of gases selected from HCl, HF, HBr, Cl2, F2 and Br2 is added to O2 atmosphere is used as the oxidizing atmosphere containing the halogen element. In these inventions, the temperature of the second heat treatment is preferable to be higher than that of the first heat treatment.
Further, it is possible to anneal in the plasma atmosphere containing oxygen and hydrogen after eliminating the thermal oxide film in these inventions. Further, the concentration of oxygen contained within the amorphous silicon film is 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923.
According to one aspect of the invention described above in (16), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, an active layer of the semiconductor device is formed by patterning the crystal silicon film and laser light or intense light is irradiated to the active layer. After that, a second heat treatment is implemented within an oxidizing atmosphere containing a halogen element to eliminate or reduce the metal element existing within the active layer. Then, a thermal oxide film formed here is eliminated and another thermal oxide film is formed on the surface of the active layer by implementing another thermal oxidation.
According to one aspect of the invention described above in (17), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, an active layer of the semiconductor device is formed by patterning the crystal silicon film and laser light or intense light is irradiated to the active layer. After that, a second heat treatment is implemented within an oxidizing atmosphere containing a halogen element to eliminate or reduce the metal element existing within the active layer. Then, a thermal oxide film formed here is eliminated and another thermal oxide film is formed on the surface of of active layer by implementing another thermal oxidation. At this time, the active layer is formed so as to have an inclined shape in which an angle formed between a side face and an underlying face is 20xc2x0 to 50xc2x0.
In the aspects of the invention described above in (16) and (17), the gate insulating film may be formed by utilizing said another thermal oxide film. Further, it is preferable that the temperature of the first and second heat treatments is below 750xc2x0 C. Preferably, the atmosphere in which one or a plurality of types of gases selected from HCl, HF, HBr, Cl2, F2 and Br2 is added to O2 atmosphere is used as the oxidizing atmosphere containing the halogen element.
In these inventions, the temperature of the second heat treatment is preferable to be higher than that of the first heat treatment. Further, it is possible to anneal in the plasma atmosphere containing oxygen and hydrogen after eliminating the thermal oxide film in these inventions. Still more, the concentration of oxygen contained within the amorphous silicon film is 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923.
The main aspects of the present invention described above in (18) through (22) are as follows.
According to one aspect of the invention described above in (18), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, laser light or intense light is irradiated to the crystal silicon film and a second heat treatment is implemented within an oxidizing atmosphere to eliminate or reduce the metal element existing within the crystal silicon film. Then, a thermal oxide film formed in that step is eliminated and another thermal oxide film is formed on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
According to one aspect of the invention described above in (19), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, laser light or intense light is irradiated to the crystal silicon film to diffuse the metal element existing within the crystal silicon film. After that, a second heat treatment is implemented within an oxidizing atmosphere to getter the metal element existing within the crystal silicon film to a thermal oxide film to be formed. Then, the thermal oxide film formed in that step is eliminated and another thermal oxide film is formed on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
According to one aspect of the invention described above in (20), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is intentionally and selectively introduced to the amorphous silicon film. As modes for selectively introducing the metal element to the amorphous silicon film, various methods may be adopted, such as 1) introducing to one end portion of the amorphous silicon film, 2) introducing to one end portion of the amorphous silicon film leaving a space, and 3) introducing in dots on the whole surface of the amorphous silicon film leaving spaces therebetween. Thereby, the crystal is grown by the following first heat treatment in a direction parallel to the film from the region to which the metal element has been selectively introduced.
That is, the first heat treatment is implemented to the amorphous silicon film to grow the crystal in the direction parallel to the film from the region to which the metal element has been intentionally and selectively introduced. Next, laser light or intense light is irradiated to the crystal silicon film to diffuse the metal element existing within the crystal silicon film. After that, a second heat treatment is implemented within an oxidizing atmosphere to getter the metal element existing within the crystal silicon film to a thermal oxide film to be formed. Then, the thermal oxide film formed in that step is eliminated and another thermal oxide film is formed on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
In the aspects of the invention described above in (19) and (20), the second heat treatment is preferably conducted in temperature above 600xc2x0 C. and below 750xc2x0 C. The gate insulating film may be formed by utilizing said another thermal oxide film. Further, it is preferable that the temperature of the second heat treatment is higher than that of the first heat treatment. still more, in these inventions, it is possible to anneal in the plasma atmosphere containing oxygen and hydrogen after eliminating the thermal oxide film. Further, preferably, the concentration of oxygen contained within the amorphous silicon film is 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923.
According to one aspect of the invention described above in (21), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is introduced intentionally to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, the crystal silicon film is patterned to form an active layer of the semiconductor device and laser light or intense light is irradiated to the active layer. After that, a second heat treatment is implemented within an oxidizing atmosphere to eliminate or reduce the metal element existing within the active layer. A thermal oxide film formed in that step is eliminated and another thermal oxide film is formed on the surface of the active layer by implementing another thermal oxidation.
According to one aspect of the invention described above in (22), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is introduced intentionally to the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, the crystal silicon film is patterned to form an active layer of the semiconductor device and laser light or intense light is irradiated to the active layer. After that, a second heat treatment is implemented within an oxidizing atmosphere to eliminate or reduce the metal element existing within the active layer. A thermal oxide film formed in that step is eliminated and another thermal oxide film is formed on the surface of the active layer by implementing another thermal oxidation. At this time, the active layer is formed so as to have an inclined shape in which an angle formed between a side face and an underlying face thereof is 20xc2x0 to 50xc2x0.
In the aspects of the invention described above in (21) and (22), the gate insulating film may be formed by utilizing said another thermal oxide film. The second heat treatment is preferably conducted in temperature above 600xc2x0 C. and below 750xc2x0 C. Further, it is preferable that the temperature of the second heat treatment is higher than that of the first heat treatment. Still more, in these inventions, it is preferable to anneal in the plasma atmosphere containing oxygen and hydrogen after eliminating the thermal oxide film. Preferably, the concentration of oxygen contained within the amorphous silicon film is 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923.
The main aspects of the inventions described above in (23) through (25) are as follows.
According to one aspect of the invention described above in (23) , an amorphous silicon film is formed on a substrate having an insulating surface and a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film. Then, the amorphous silicon film is crystallized by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C. to obtain a crystal silicon film. The crystal silicon film is patterned to form an active layer of the semiconductor device.
After that, a second heat treatment is implemented within an oxidizing atmosphere containing a halogen element to eliminate or reduce the metal element existing within the crystal silicon film. A thermal oxide film formed in the previous step is eliminated and another thermal oxide film is formed after eliminating the thermal oxide film by implementing another thermal oxidation. At this time, the heat treatments are implemented such that a temperature of the second heat treatment is higher than that of the first heat treatment.
According to one aspect of the invention described above in (24) , an amorphous silicon film is formed on a substrate having an insulating surface and a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film. Then, the amorphous silicon film is crystallized by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C. to obtain a crystal silicon film. The crystal silicon film is patterned to form an active layer of the semiconductor device. After that, a second heat treatment is implemented within an oxidizing atmosphere containing a halogen element to getter the metal element existing within the active layer to a thermal oxide film to be formed. The thermal oxide film formed in the previous step is then eliminated and another thermal oxide film is formed after eliminating the thermal oxide film by implementing another thermal oxidation. At this time, the heat treatments are implemented under the condition that a temperature of the second heat treatment is higher than that of the first heat treatment.
According to one aspect of the invention described above in (25), an amorphous silicon film is formed on a substrate having an insulating surface and a metal element which promotes crystallization of silicon is intentionally introduced to the amorphous silicon film. As modes for selectively introducing the metal element to the amorphous silicon film, various methods may be adopted, such as 1) introducing to one end portion of the amorphous silicon film, 2) introducing to one end portion of the amorphous silicon film leaving a space, and 3) introducing in dots on the whole surface of the amorphous silicon film leaving spaces therebetween. Thereby, the crystal is grown by the following first heat treatment in a direction parallel to the film from the region to which the metal element has been selectively introduced.
That is, the crystal is grown by the first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C. in a direction parallel to the film from the region to which the metal element has been intentionally and selectively introduced. Then, the crystal silicon film is patterned to form an active layer of the semiconductor device by using the region in which the crystal has grown in the direction parallel to the film. After that, a second heat treatment is implemented within an oxidizing atmosphere containing a halogen element to getter the metal element existing within the active layer to a thermal oxide film to be formed. The thermal oxide film formed in the previous step is then eliminated and another thermal oxide film is formed after eliminating the thermal oxide film by implementing another thermal oxidation. At this time, the heat treatments are implemented under the condition that a temperature of the second heat treatment is higher than that of the first heat treatment.
In the aspects of the invention described above in (23) through (25), preferably, a quartz substrate is used as the substrate for forming the amorphous silicon film and the gate insulating film is formed by utilizing said another thermal oxide film. Further, in these inventions, it is possible to anneal in the plasma atmosphere containing oxygen and hydrogen after eliminating the thermal oxide film. Still more, preferably, the concentration of oxygen contained within the amorphous silicon film is 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923.
The main aspects of the present invention described above in (26) through (29) are as follows.
According to one aspect of the invention described above in (26), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is held in contact on the surface of the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, a second heat treatment is implemented in the temperature range of 500xc2x0 C. to 700xc2x0 C. within an atmosphere containing oxygen, hydrogen and fluorine to form a thermal oxide film on the surface of the crystal silicon film. Then, the thermal oxide film is eliminated.
According to one aspect of the invention described above in (27), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is held in contact on the surface of the the amorpnous silicon film and the amorphous silicon film is crystallized. by a first heat treatment to obtain a crystal silicon film. Next, a second. heat treatment is implemented in the temperature range of 500xc2x0 C. to 700xc2x0 C. within an atmosphere containing oxygen, hydrogen, fluorine and chlorine to form a thermal oxide film on the surface of the crystal silicon film. Then, the thermal oxide film is eliminated.
According to one aspect of the invention described above in (28), an amorphous silicon film is formed at first. Then, a metal element which promotes crystallization of silicon is held in contact on the surface of the amorphous silicon film and the amorphous silicon film is crystallized by a first heat treatment to obtain a crystal silicon film. Next, a second heat treatment is implemented within an atmosphere containing fluorine and/or chlorine to form a wet oxide film on the surface of the crystal silicon film. Then, the oxide film is eliminated.
In the aspects of the invention described above in (26) through (28) above, preferably the concentration of the metal element within the oxide film is higher than that of the metal element within the crystal silicon film. Further, it is preferable to contain more than 1% and below an explosion limit of hydrogen in the atmosphere in which the second heat treatment is implemented. Further, it is preferable to implement the first heat treatment in a reducing atmosphere. Laser light may be irradiated to the crystal silicon film after the first heat treatment.
According to one aspect of the invention described above in (29), there is provided a semiconductor device having a silicon film having a crystallinity, characterized in that the silicon film contains a metal element which promotes crystallization of silicon in concentration of 1xc3x971016 cmxe2x88x923 to 5xc3x971018 cmxe2x88x923, fluorine atoms in concentration of 1xc3x971015 cmxe2x88x923 to 1xc3x971020 cmxe2x88x923, and hydrogen atoms in concentration of 1xc3x971017 cmxe2x88x923 to 1xc3x971021 cmxe2x88x923. In this invention, preferably, the silicon film is formed on the insulating film and fluorine atoms exist in high concentration near the interface between the insulating film and the silicon film.
The main aspects of the invention described above in (30) through (33) are as follows.
According to one aspect of the invention described above in (30), an amorphous silicon film is formed at first and the amorphous silicon film is crystallized to form a crystal silicon film. Next, this crystal silicon film is heated within an oxidizing atmosphere to which fluorine compound gas is added to grow a thermal oxide film on the surface of the crystal silicon film. Then, the thermal oxide film on the surface of the crystal silicon film is eliminated. After that, an insulating film is deposited on the surface of the crystal silicon film to complete the fabrication of the semiconductor device.
According to one aspect of the invention described above in (31), there is provided a method for fabricating a thin film transistor on a substrate having an insulating surface. An amorphous silicon film is formed at first and the amorphous silicon film is crystallized to form a crystal silicon film. Next, this crystal silicon film is heated within an oxidizing atmosphere to which fluorine compound gas is added to grow a thermal oxide film on the surface of the crystal silicon film. Then, the thermal oxide film on the surface of the crystal silicon film is eliminated. After that, the crystal silicon film is shaped to form an active layer of the thin film transistor and an insulating film is deposited on the surface of the active layer to form a gate insulating film at least on the surface of a channel region. Further, a gate electrode is formed on the surface of the gate insulating film and impurity ions which give a conductive type are injected into the active layer by using the gate electrode as a mask to form a source and a drain in a manner of self-alignment.
According to one aspect of the invention described above in (32) , an amorphous silicon film is formed and the amorphous silicon film is crystallized by irradiating laser light to form a crystal silicon film. Next, this crystal silicon film is heated within an oxidizing atmosphere to which fluorine compound gas is added to grow a thermal oxide film on the surface of the crystal silicon film. Then, the thermal oxide film on the surface of the crystal silicon film is eliminated. After that, an insulating film is deposited on the surface of the crystal silicon film to complete the fabrication of the semiconductor device.
According to one aspect of the invention described above in (33), there is provided a method for fabricating a thin film transistor on a substrate having an insulating surface. An amorphous silicon film is formed at first and the amorphous silicon film is crystallized to form a crystal silicon film. Next, laser light is irradiated to the crystal silicon film and the is heated within an oxidizing atmosphere to which fluorine compound gas is added to grow a thermal oxide film on the surface of the crystal silicon film. Then, the thermal oxide film on the surface of the crystal silicon film is eliminated.
Next, the crystal silicon film is shaped to form an active layer of the thin film transistor, an insulating film is deposited on the surface of the active layer to form a gate insulating film at least on the surface of a channel region and a gate electrode is formed on the surface of the gate insulating film. Further, a source and a drain are formed in a manner of self-alignment by injecting impurity ions which give a conductive type to the active layer by using the gate electrode as a mask. Thus, the semiconductor device is fabricated.
In the aspects of the invention described above in (30) through (33), preferably, the thickness of the thermal oxide film is 200 to 500 angstrom and the metal element is doped to the amorphous silicon film after forming the amorphous silicon film in concentration of 1xc3x971016 to 5xc3x971019 atoms/cm3. Further, while a metal element is preferable to use in forming the crystal silicon film, at least one or more types of elements selected from Fe, To, Ni, Ru, Rh, Pd, Os, Ir, Cu and Au may be used as the metal element similarly to the aspects of the invention described above.
The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description of the preferred embodiments and the accompanying drawings.